1. Field of the Invention
This invention relates to a method of stabilizing an alkaline aqueous solution of thiourea dioxide.
2. Description of the Prior Art
Thiourea dioxide, which is also called aminoiminomethanesulfinic acid or formamidinesulfinic acid, is sold on the market industrially and is available as a white powder superior in preservative stability and having neither oxidizing property nor reducing property.
Thiourea dioxide displays reducing property when an aqueous solution thereof is made alkaline or heated, and its reducing power is very large. Besides, as compared with conventional reducing agents, e.g. sodium hydrosulfite, thiourea dioxide as powder or aqueous solution is superior in stability and scarcely produces a bad smell.
Such characteristic features of thiourea dioxide allow this substance to be used in various fields, including its application to the textile industry, for example as a reducing agent for vat dyes, a reduction clearing agent for fibers dyed with disperse dyes, a decoloring agent for fibers dyed with various dyes, a tank detergent for dyeing machines, a shrink-proofing agent for keratin fibers, a bleaching agent for protein fibers, polyamide fibers and phenolic resin fibers, a decolorizing agent to be used in the manufacturing process for polyacrylonitrile fibers and polyvinyl alcohol fibers, a white discharge printing agent for various dyes, a colored discharge printing agent, and a color fastness improver; and also its application as a pulp bleaching agent, an antioxidant for organic amines, a polymerization catalyst, a photographic sensitizing aid, an ingredient of cleaning materials, a reducing agent for metal ions, and reducing agents of organic compounds, for example as nitro compounds to hydrazo compounds or amines, ketones to secondary alcohols, aldehydes to primary alcohols, and disulfides to thiols.
Thiourea dioxide is in many cases used as an alkaline aqueous solution to display its reducing power effectively. And as alkalis there are used from strong alkalis such as caustic soda and caustic potash up to even alkali salts of weak acids such as phosphoric acid, polymerized phosphoric acid, carbonic acid, boric acid and organic acid. However, an aqueous solution of thiourea dioxide becomes easily decomposable with increasing strength of alkali. For example, a solution of thiourea dioxide dissolved in a concentrated solution of caustic soda which is one of strong alkalis decomposes gradually to a larger extent when left standing for a long time even at room temperature, and its reducing power becomes lower. Thus, in a strong alkali solution the use of thiourea dioxide often causes troubles in point of practical application, though its use in a weak alkali solution does not bring about so much decomposition thereof and so scarcely causes problem in practical application. In case thiourea dioxide and a strong alkali are dissolved together in advance and this solution is used little by little, the reducing power of the solution just after preparation differs from that after a certain elapse of time, and in the latter case it is required to use an extra amount of the solution in order to obtain the same effect.
To solve such a problem there have heretofore been adopted a method in which the solution is made concentrated beforehand in anticipation of decomposition, a method in which the solution only in a required amount is prepared just before use, and a method in which thiourea dioxide as powder is fed to a predetermined place. However, all these methods involve problems in point of economy, work and environment.